Refuse to start if log_filename points to a non-existing directory
Refuse to start if log_filename points to a non-existing directory

Closes #189

UCL-ShippingGroup/Windshaft-cartodb,manueltimita/Windshaft-cartodb,manueltimita/Windshaft-cartodb,CartoDB/Windshaft-cartodb,manueltimita/Windshaft-cartodb,CartoDB/Windshaft-cartodb,CartoDB/Windshaft-cartodb,UCL-ShippingGroup/Windshaft-cartodb,CartoDB/Windshaft-cartodb,UCL-ShippingGroup/Windshaft-cartodb